agent: Improve discoverability of the quote selection action (#40897)

This PR renames the `agent::QuoteSelection` to
`agent::AddSelectionToThread` _and_ adds it as a menu item in both the
right-click context menu within regular buffers as well as the
"Selection" app menu.

We've received feedback in the past about how hard to discover this
feature is, and after watching [the Syntax podcast
crew](https://www.youtube.com/watch?v=bRK3PeVFfVE) recently struggle
with doing so—and then naturally looking for it in the context menu and
not finding it—it felt like time to push a change. I think the rename +
the availability in these places could help bringing it to surface more.

The same action can be done in Cursor through the `cmd-l` keybinding,
but in Zed, that triggers `editor::SelectLine`, which I don't want to
override by default. However, if you're using Cursor's keymap, then
`cmd-l` does trigger this action, as expected.

<img width="500" height="1812" alt="Screenshot 2025-10-22 at 12  01@2x"
src="https://github.com/user-attachments/assets/dfc2c41c-8d0a-4a1a-8ea1-1bd5d1aa1171"
/>


Release Notes:

- agent: Improves discoverability of the previously called "quote
selection" action—which allows to add a text selection in a buffer as
context within the agent panel—by renaming it to "add selection to
thread" and making it available from the right-click editor context menu
as well as the "Selection" app menu.

assets/keymaps/linux/cursor.json

@@ -17,8 +17,8 @@
     "bindings": {
       "ctrl-i": "agent::ToggleFocus",
       "ctrl-shift-i": "agent::ToggleFocus",
-      "ctrl-shift-l": "agent::QuoteSelection", // In cursor uses "Ask" mode
-      "ctrl-l": "agent::QuoteSelection", // In cursor uses "Agent" mode
+      "ctrl-shift-l": "agent::AddSelectionToThread", // In cursor uses "Ask" mode
+      "ctrl-l": "agent::AddSelectionToThread", // In cursor uses "Agent" mode
       "ctrl-k": "assistant::InlineAssist",
       "ctrl-shift-k": "assistant::InsertIntoEditor"
     }